Video mixers, known also as production switchers, are used for combining and manipulating video signals from a number of video sources to form one or more output video signals comprising selections from, and combinations of, the video inputs. In live production the mixer operator controls the transition from one shot to the next; facilities for the combination and selection of audio signals may also be provided in the same equipment.
Typically an operator is able to select a video signal by pressing one of a row of push buttons, known as a “bus”. Mixers usually have between two and eight such busses grouped in pairs, which are associated with a particular video output. This output consists of one of the bus selections, or a combination of the selections from the two paired busses. The operator typically changes the output from one bus to the other, and thus the output picture from one picture to another picture, by moving a fader handle, often known as a T-Bar. The transition may be, for example, a cross-fade, or a wipe. In a cross-fade, the proportions of the two bus signals depend on the position of the fader. In a wipe both signals are present at full amplitude in the output but a variable portion of one picture is removed to reveal the other picture. The shape of the revealed portion is pre-determined by the operator and the relative areas of the output picture contributed by the respective input pictures depend on the position of the fader.
Other types of combination of, and transition between, pairs of busses are known and these may involve the use of signals from other busses to modify the transition or combination. A common method of combining pictures is the technique of “keying” in which the brightness or colour of a part of a picture is used to control the substitution of that part of that (“foreground”) picture for the equivalent part of another (“background”) picture. Alternatively the controlling, or key, signal may be a third signal different from (but usually related to) the foreground or the background.
Video mixers may not necessarily be controlled directly by an operator. Semi-automatic systems exist in which the operator initiates a video transition from one shot to another (e.g. between one bus and another), but the type of transition and the length of time needed for the transition to occur have been pre-programmed. Similarly, fully automatic systems are known in which all aspects of the video transitions are under the control of an automation system.
It is well-known to include a video store in a video mixer system, for example a solid state memory device holding one or more frames of video. If several frames can be stored, it is then possible for a dynamic video sequence to be played in response to an action by the operator, such a sequence is often referred to a “clip” by analogy with a short length of film.
Typically the sequence may be started when the operator moves the fader handle to initiate a transition from one video source to another. For example, as the edge of a wipe transition passes across the output picture, a moving logo, played from a store, may be keyed over the wipe.
Although it is possible to initiate the playing of the clip of the logo from the start of the fader movement controlling the wipe transition, the progress of the logo is determined by its original recording, whereas the progress of the wipe transition is controlled by the operator. The operator may wish to modify the progress of the wipe transition in real time in response to the video content, which may be live and therefore unpredictable.